EVM Node: Add GCP KMS module interface documentation

What

Added comprehensive documentation to the gcp_kms.mli interface. This includes descriptions for types t and hash_algorithm, and functions like from_gcp_key, gcp_key, public_key, sign, and ethereum_address_opt.

Why

To improve clarity and understanding of the GCP KMS module's public interface. To provide detailed explanations on how to use the functions, their expected inputs and outputs, and any specific behaviors or limitations, such as the digest algorithm note for sign. This enhances maintainability and makes it easier for developers to interact with the module correctly.

How

OCaml documentation comments were added directly into the .mli file. All public types and functions now have clear, concise, and accurate descriptions. Specific details were included regarding supported key types for from_gcp_key and the implications of using different hash algorithms for sign.

Manually testing the MR

Read and enjoy.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR